Add Metadata class and method docstrings

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

tuf/api/metadata.py

@@ -58,13 +58,26 @@
 # Classes.
 
 class Metadata():
+ """A container for signed TUF metadata.
+
+ Provides methods to (de-)serialize JSON metadata from and to file
+ storage, and to create and verify signatures.
+
+ Attributes:
+ signed: A subclass of Signed, which has the actual metadata payload,
+ i.e. one of Targets, Snapshot, Timestamp or Root.
+ signatures: A list of signatures over the canonical JSON representation
+ of the value of the signed attribute.
+
+ """
  def __init__(
  self, signed: 'Signed' = None, signatures: list = None) -> None:
  # TODO: How much init magic do we want?
  self.signed = signed
  self.signatures = signatures
 
  def as_dict(self) -> JsonDict:
+ """Returns the JSON-compatible dictionary representation. """
  return {
  'signatures': self.signatures,
  'signed': self.signed.as_dict()